Background & objectives: Sulfur B dye is one of the used dyes in the textile industry which is disposed into the wastewater in the large amount during textile industry activities. In this research, the photocatalytic degradation of sulfur B dye was investigated using ZnO as a photocatalyst under UV irradiation in aqueous solutions.Methods: In this experimental study, a 2.8 L reactor is used. The effect of operational factors including photocatalyst (0.1-4000 mg/l), dye concentration (1-400 mg/l) and the initial pH (3-11) were investigated in the steady UV intensity on dye removal. Dye concentration was measured by spectrophotometer at 550 nm wavelength.Results: The result of this study reveales that by increasing the pH from 3 to 11, the efficiency of dye removal increases from 28 to 90%, respectively. Increment of the initial dye concentration from 1 to 400 mg/l reduces the dye removal from 94 to 57%, respectively. By increasing catalyst to initial dye up to 3 mg/l, the dye removal increases to 94% without any further dye removal efficiency by adding the ratio. Conclusion: The results showed that concurrent existence of catalyst and UV is necessary for further dye removal. Increasing pH value can produce higher hydroxyl radical, which can increase dye removal. Increasing initial dye concentration prevents efficient light penetration through the solution and thus dye removal is reduced. Increasing the amount of nanoparticle dosage increases the presence of active sites to produce more hydroxyl radicals.